The processing of ultra high molecular weight polyethylene (UHMWPE), i.e. polyethylene having a molecular weight in excess of 5 million, is known in the polymer arts to be extremely difficult. Products made from such materials are, however, very strong, tough and durable.
In the following series of U.S. patents filed by Kobayashi et al and assigned to Nippon Oil Co., Ltd. a number of inventions related to the fabrication of fibers and films of polyolefins generally and UHMWPE specifically, are described: U.S. Pat. Nos. 4,996,011, 5,002,714, 5,091,133, 5,106,555, 5,200,129, and 5,578,373. The processes described in these patents are depicted schematically in FIG. 1 and generally describe the continuous production of high strength and high modulus polyolefin films by feeding polyolefin powder between a combination of endless belts disposed in an up and down opposing relationship, compression molding the polyolefin powder at a temperature below its melting point between the endless belts and then rolling and stretching the resultant compression molded polyolefin into an oriented film. Some of these patents also discuss the fibrillation of the resultant films and slitting of the films to form “fibers”. As compression molded, the sheet is relatively friable thus requiring the subsequent stretching or drawing operations to provide an oriented film that exhibits very good strength and durability properties. In fact, the strength of such materials produced by these processes is 3 times that of steel on a weight basis and they exhibit very low creep. The UHMWPE films produced by the processes described in these patents have a final thickness of between 0.003″ and 0.012″.
While the thus produced materials quite obviously exhibit highly desirable properties, including useful ballistic properties, one of their major shortcomings is their relative stiffness that makes them difficult to “weave” and otherwise process into useful products. When woven, the resulting fabrics also tend to be very stiff and uncomfortable. This stiffness is largely a result of the fact that the fibers or tapes produced as just described are relatively “thick”, i.e. on the order of more than about 3 mils. In order to obtain a material that can be easily woven to provide comfortable clothing and the like, and find use in such other applications as dental floss (another “high strength/thin material application) and high strength “thread” or fiber, it is necessary that the “thickness” of the UHMWPE film be reduced to below 3 mils and preferably below about 2 mils. Subsequent slitting and other treatments, for example fibrillation, can further contribute to the production of such products. Because of the high strength of these materials, it has been thought until now that the best approach to achieve such “thickness reduction” was to slit the film of the prior art into narrow strips (on the order of about 10 mils) and to stretch such narrow strips. This has proven largely unsuccessful since the material in such narrow widths will either refuse to stretch or break when elongated under conventional drawing and/or calendering processes. In fact, until the application of the methods described herein, to the best of our knowledge, no attempt to achieve draw ratios greater than 100, as would be necessary to meet the above-described requirements, has been successful. It is well known in the art that at a given thickness, the ballistic performance of an assembly is enhanced by having more layers due to the ability of multiple thinner layers acting in concert function more efficiently in ballistic applications.
It therefore would be most desirable to define a process whereby these high strength materials could be fabricated into films, sheets or tapes and “fibers” that are less than 3 mils and preferably below about 2 mils in thickness. The provision of such a process would open up entirely new applications for these materials in such diverse fields as ballistic structures.